Imaging Upper Mantle Discontinuities Using Long Period Seismic Data

• Author / Creator

  Mohammad Hosseini Dokht, Ramin

• Mantle reflectivity structure provides critical information on the temperature and composition of the mantle. So far, these structures are obtained by independent analyses of amplitudes and timing information of reflected and converted seismic phases at mantle discontinuities. These approaches are limited due to their inherent trade-off between velocity and gradient zone depth. This thesis aims to provide high resolution, novel imaging methods based on a nonlinear inversion method and wave spectral characteristics. Substantially improved data constraints, combined with increased imaging resolution and accuracy from this thesis, offer new insights into the process and effects of subduction (both present and past) on mantle stratification, fluid content and magmatism along the major convergent margins. We explore the regional variations of the 410 km and 660 km discontinuities by careful analyses of the underside reflected S-wave energy (SS precursors) from mantle interfaces using nonlinear, simultaneous inversions of shear velocity and discontinuity topography. We find that the 410 km and 660 km discontinuity depths are strongly anti-correlated if the dipping angles of the non-vertical structures were taken into account. Beneath the volcanic centers in Northeast China, our simultaneous solutions provide compelling evidence for the source of deep-rooted mantle melting in association with the dehydration process of the stagnant Pacific slab. An independent study using Singular Spectrum Analysis (SSA) for random noise removal and reconstruction of missing traces shows a significant enhancement of signals associated with mantle conversions beneath southwestern Canada. In addition to strong conversions from the transition zone phase boundaries, strong signals from the mid-transition zone reflectors can also be identified. Finally, our results from pre-stack depth migration of precursory arrivals reveal small-sale variations (< 500 km) on the discontinuity topography, due to the focusing of the diffracted energy to its true position. In Southeast Asia, the discontinuity depth measurements indicate maximum undulations of ~ 40 km on both transition zone discontinuities along the Sunda and Banda arcs. The correlation between the discontinuity depths and previously reported seismic velocities suggests that the upper mantle phase boundaries are thermally, rather than compositionally, controlled.

• Subjects / Keywords

  • Migration Imaging
  • SS precursors
  • Waveform Inversion
  • Mantle Transition Zone
  • Subduction Zones
  • Signal Processing

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3CR5NR0G

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.